The present invention relates to a slip-controlled brake system for all-wheel drive motor vehicles furnished with differentials for compensating position differences between the individual wheels, and comprising wheel sensors and electrical circuits for generating brake pressure control signals suitable for control of the brake pressure in the event of imminent locking.
The state-of-the-art brake slip control systems, primarily, are designed for motor vehicles having only one driven axle. In measuring the rotating characteristics, of each wheel and analyzing the measured results, specifically, in the logic operation of the measured values and in the control of th progress of the brake pressure, consideration has been given as to whether it is a driven or a non-driven wheel. Special importance is attached to the distinction between driven and non-driven wheels in the formation of the vehicle reference speed with which is compared the wheel rotating characteristics of the individual wheels. This reference speed serves as a reference figure for controlling the brake pressure progress of the individual wheels during the slip control (DE-Offenlegungsschrift No. 22 54 295).
Adaptation of the conventional brake slip control systems to all-wheel drive motor vehicles, hence, involves substantial difficulties that are further increased in the event that clutches forming differential locks are assembled into the differentials, especially into the rear axle differential and into the intermediate differential. The rigid coupling between the wheels and between the front axle and the rear axle, by way of the drive train, results in a mutual influence over all of the wheels, whereby, changes in the brake force and/or in the friction force, respectively, between tire and roadway, no longer can be detected by measuring the rotating characteristics of the individual wheels and a logical analysis of the measured values. The locked coupling, by way of the drive train, in a variety of situations results in an almost synchronous characteristics of all wheels so that it will, for example, not be possible to identify the first wheel becoming unstable or to perform a control according to the select-low principle.
In prior all-wheel drive vehicles utilizing lockable differentials it has been imperative to render inoperative the slip control when engaging the differential locks. This involves the disadvantage that, specifically, in unfavorable road or wheather conditions in which the differential locks are useful, the slip control has to be foregone, eventhough the slip control, specifically in snow and ice conditions where unbalanced friction values are present and the slip control provides special advantages which are preferable and required for maintaining the steerability and the driving stability of the vehicle.
It is, therefore, the object of the present invention to overcome the described disadvantages involved with the prior art brake slip control systems designed for all-wheel drive vehicles and to improve such brake systems to the effect that the brake slip control of the all-wheel driven vehicle can be put into operation in whatever situation and, depending on the number of control channels, said system permits an individual control of the single wheels or wheel groups.